Case Study on Ground Improvement

by High Pressure Jet Grouting

Hong, Won-Pyo, Kim, Dong- Wook, Lee, Mm-Ku, Yea, Geu-Guwen

Department of Civil and Environmental Engineering, Chung-Ang University, Seoul, Korea

ABSTRACT

If excavation works in front of a temporary retaining wall is performed in loose sand with high ground water level, a boiling problem would be induced at the site. Due to excessive ground settlement mduced by the boiling at excavation bottom, severe damage may take place to structures in the vicinity of an excavation site. Recently, in order to cutoff ground water behind a temporary retaining wall during excavation, the high pressure jet grouting has been widely used in Korea. The purpose of this study is to investigate, first, the engineering properties of the ground improved by the high pressure jet grouting method, then, to confirm in field the effects of cutoff of ground water behind temporary retaining walls during excavation. A series of laboratory and field tests is performed to investigate the effect of ground improvement; unconfined compression tests are performed on the specimens sampled from grouted ground and permeability tests are performed in bore holes drilled in the grouted ground. Test results indicate that the cutoff characteristics of the grouted ground is affected by the water content of the original ground, the jet nozzle pressure, the type of jetting method and the ground water level. This study also shows that the high pressure jet grouting has a sufficient e&iency to cutoff ground water at excavation works.

KEY WOIRDS : high pressure jet grouting, boiling, coefficient of permeability, cutoff of ground water, ground improvement, cohesionless; soil

INTRODTJCTION

Occasionally, subway construction in Korea is carried out in an alluvial soil layer with high ground water level and high permeability. When an excavation work is applied to these poor soils, boiling would be induced at the excavation bottom surface due to the difference in ground water levels between the excavation region and the ground behind temporary retaining walls (Hong et al, 1992). When a boiling occurs to these poor soils, ground surface settlement at excavation sites and cracking in the vicinity of neighboring

structures may take place, because soil particles behind the temporary retaining walls flow away with the void water. The high pressure jet grouting method is generally applied so as to eliminate the settlement problem or obtain the cutoff effects of the ground water behind the retaining wall for excavation. This method has been widely used for installing circular soil-cement mixture columns behind the temporary retaining wall or for constructing a cutoff wall under a dam (Miyasaka et al 1986; R. H. Borden; R. D. Holtz and I. Juran). Especially, the high pressure jet grouting method can restrict the excessive horizontal displacement of a temporary retaining wall, because the earth pressure acting on the retaining wall can be reduced during excavation. Before excavation, improving the ground helps prevent excessive deformation of ground by restraining the heaving or boiling at excavation bottom surface. On the other hand, this method is also widely adopted for foundation reinforcement such as underpinning works, protection of old buildings and reinforcing the bearing capacity of soft ground (Ichihashi et al. 1985). The purpose of this study is to investigate the cutoff effect of a grouted ground wall installed by the high pressure jet grouting method behind a temporary retaining wall. The investigation is performed on three temporary retaining wall construction sites with high ground water level in silty sand and sandy gravel layers.

Particularly, comparing the cutoff effect of ground water in ground

grouted by high pressure jet with that in case of applying low

pressure jet, the high efficiency of the high pressure jet grouting is

observed.

GROUTING SITES

I. Field description Three sites chosen for this study were intended to be reinforced by grouting for the purpose of obtaining the efftcient cutoff of ground water behind temporary retaining walls. Various grouting methods were applied in the sites. In Site-l, soil was mixed in ground by cement with relatively low pressure jet of 5 to 10 kgf/cm* to make soil cement columns in subground. The soil cement columns are overlapped in a row so as to form a wall in subground.

Proceedings of The Twelfth (2002) International Offshore and Polar Engineering ConferenceKitakyushu, Japan, May 26–31, 2002Copyright © 2002 by The International Society of Offshore and Polar EngineersISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set)

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Table 1. The grouting method and the adapted pressure

I Sites

Grouting methods

Adapted pressures (k&cm’)

Diameter of grouted columns (cm)

Site-l

Soil cement wall

5-10

Site-2 Site-3

High pressure jet grouting

200 450

120

This grouting method is called by the soil cement wall grouting method. Besides, in Site-2 and Site-3, soil cement columns were constructed by injection of cement with high pressure jet in subground. The high pressure jet could be supplied through center tube rod to inject cement with air and water to soil. The center tube rod applied in Site-2 was the double tube rod type, while the triple tube rod type was applied in Site-3. This grouting method is called by the high pressure jet grouting method. The jet nozzle pressure through the triple tube rod was 450 k&cm* in Site-3. In Site-2 constructed by double tube rod type, the jet nozzle pressure was 200 kgfYcmz as shown in Table 1. In Site-l, excavation was performed without any grouting at first. The excavation could not be continued below the ground water level because of :severe boiling problems during excavation. Therefore, the soil celment wall grouting method was adopted behind the retaining w,all as a countermeasure. However, the boiling was continued. Consequently, the method failed to obtain the cutoff effect of ground water. Photo 1 shows the circular shaped fountain holes caused1 by boiling at Site-l.

Photo 1. Boiling in Site-l

The other sites were also located at ground excavation sites for subway construction in alluvial soils, in which Site-2 was located at the 7-23 sector in the Seoul subway line 7 and Site-3 was located at the 6-2 sector in the Seoul subway line 6. Excavation in these sites would make the adjacent apartment buildings unstable, because of the draw down of ground water level and the co.nsiderable horizontal displacement of the temporary retaining wall1 due to excavation. The high pressure jet grouting

method was adopted to obtain the cutoff effect of the ground water behind the temporary retaining wall. After the grouting, excavation could be completed without boiling problems. Consequently, the effective cutoff could be obtained in these sites.

Figure 1. Site location

2. Ground profile Figure 2 shows soil profiles in the three sites with the results of standard penetration tests. Soil profiles show a little difference among the three sites, but the general pattern of soil profiles shows that the vertical soil distribution is fill, alluvial soil, weathered zone and bedrock from ground surface in turn.

The topsoil consists of a fill layer composed of a mix of silt, clay,

sand and a small amount of gravel mainly. This layer was formed in

the course of earth works to construct a road and a new city.

Relative density of the fill layer is identified to be loose state. An alluvial soil layer deposited for a long-term period exists below the fill layer and is composed of deposit materials removed by overflow from upstream floods. And the alluvial soil is divided into silty clay, silty sand, sand and sandy gravel. Relative density of the upper alluvial soil is identified as a state of medium dense descending to a dense state. Especially, the silty sand and sandy gravel at these sites make the aquifer, because the sites belong to the Han-river system region. Therefore, the soils in this region has a high permeability. Weathered zone is separated into two layers as the upper weathered soil and the lower weathered rock. The weathered soil is in a completely weathered state from the bedrock, remaining as a residual soil of silt and sandy silt.

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Figure 2. Ground profiles

Weathered rock is produced by only changing its mechanical property from bedrock, remaining the texture of in-situ bedrock containing a part of discrete rock fracture. Relative density shows the range from dense to very dense state. Soft rock in the bedrock consists of the gneiss classified from Kyung- Gi metamoqphic rock in Pre-Cambrian. Sample cores am used to detect the rock fracture shapes that remarkably develops both cracks and joints. The ground water level was founded to be at a considerable height; Site-l has at G.L-l.O--4.Om, in Site-2 at G.L-3.0--3.5m and in

Site-3 at G.L-4.9m - -6.7m, respectively.

- TEST RESULTS IN FIELD

Grouting was adapted to cutoff the ground water in the three sites. When grouting is performed, it is necessary to confirm the cutoff effect in the grouted ground. Lugeon value (1933) measured by a packer test is obtained by a pump-in test, in which the volume of water taken in a section of bore hole is measured during certain time intervals. Packer tests were performed in boreholes drilled in the grouted ground in-situ to certify how much cutoff effect could be obtained. Lugeon Values are measured at the weathered rock in the case of Site-Z as 4.7 - 17 Lugeon before grouting and 4.2 Lugeon after grouting.

Table 2. The coefficient of permeability for each soil layer

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*if-8 I6 24 32 !O

Figure 4. Comparison of strength for submerged and dried cores

The coefficient of permeability for grouted ground could be estimated on the basis of the results of the packer tests. Then, the coefficient of permeability between original ground and grouted ground was compared. Table 2. presents the coefficient of permeability, which is obtained by the packer tests in the three sites. The results summarized in Table 2 indicate that the coefficient of permeability is nearly the same for each soil layers in each sites. Table 2. shows that the cutoff effect of ground water is dependent on the grouting method which is applied in the sites. The Site-l was performed by soil cement wall grouting method. According i:o permeability test, the coefficient of permeability was nearly not changed from the original value before grouting. It was

due to nozzle jet pressure adapted by 5 - 10 kgf/cm*, which is relatively lower jet pressure than any other Site-2 (200 kgfYcm* ), Site3( 450 kgfYcm* ) consequently, the grouting method with low pressure jet was not effective to cutoff ground water. Besides, the coefficient of permeability in Site-2 and Site-3, where the high pressure jet grouting was applied, were lowered considerably. Therefore, the cutoff effect of ground water in these sites would be very effective. The coefficient of permeability in the sites where the high pressure jet is applied shows the range of 1O‘5 to 106cm/sec, while the permeability is nearly not lowered from the original value in the site where the low pressure jet is applied. On the othar hand, comparing the permeability of the grouted ground in the two sites where the high jet grouting is applied, a little difference on permeability is found. That is, the lower jet pressure produces thle higher permeability, which means the lower cutoff effect of ground water. A large soil-cement mixture column can be formed in ground due to the high pressure jet grouting. The diameter of the grouted ground columns in Site-3 was 12Ocm as shown in Table I. The size of columns was measured at the depth 2.0m from the ground surface in field.

GROUTING EFFECT

1. The strength of grouted ground The strength of ground will be increased by ground improvement due to grouting. To investigate the strength of the grouted columns in subground, some cores were sampled from the grouted columns. Unconfined compression tests were performed on the cores, of which

Water content w&l

Figure 3. Relationship between unconfmed compressive strength and water content

the water content were measured in advance. The water content of the cores sampled in Site-l shows the range of 10% to 30%.

Figure 3 shows the relationship between unconfined compressive

strength and water content of the cores. This figure shows that the

unconfined compressive strength has a tendency to decline inversely

proportional to the increase of water content. That is, the higher

strength could be obtained from the cores, which have the less water

content. This result indicates that the properties of the grouted ground is affected by the water content, which is related to the ground water level in field. The grouted ground is made of the soil-cement mixture, which is different from concrete or cement mortar. That is, the grouted ground made of the soil-cement mixture contains soil particles. Therefore, the grouted ground is easy to be affected by the ground water level. The strength of the grouted ground will be varied along with the variation of ground water level during rainy and dry seasons. To investigate the effect of variation of ground water level on the strength of the grouted ground, two kinds of cores were prepared for unconfined compression tests and indirect tension tests. One is the

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cores submerged in water for 48 hours, the other is the cores dried in the oven for 24 hours. These two lcinds of cores represent the saturated condition and the dried condition. Figure 4 shows the comparison of the strength obtained from the two kinds of cores. Figure 4(a) shows the comparison of unconfined compressive strength of the saturated cores with that of the dried cores, while Figure 4(b) shows the comparison of the tensile strength. As shown in Figure 4, the unconfined compressive strength of the grouted ground for submerged condition is 20% on the average (45% at the max:imum) less than that for dry condition, while tensile strength as well as unconfined compressive strength is 20% on the average (60% at the maximum) less than that for dry condition. Consequentlly, the strength of grouted ground will be considerably affected by existence of ground water. Therefore, on application of high pressure jet grouting, the elevation or existence of ground water level should be observed accurately to maximize the effect of ground improvement.

2. The permeability of grouted ground The ground water level in each sites was located at G.L. -1 .O - -4.Om,

G.L. -3.0--3.5m and G.L. -4.9--6.7m, respectively. As mentioned in the previous section, boiling was induced during excavation in Site-l and also there were a high possibility of boiling problems inI the other two sites. The silty sand and sandy gravel found at the sites make this region an aquifer, because the sites belong to the Han-river system region. Special cutoff treatments of ground water would be necessary for excavation in these sites due to high permealbility in alluvial soils. Accordingly, a grouted ground wall was made in subground by overlapping the grouted columns constructed by high pressure jet grouting in a row for achieving a watertight barrier. Packer tests were performed in the bore holes drilled in grouted ground to investigate the permeability of the grouted ground. Table 2 shows the coefficient of permeability along with the each soil layer in both the original ground and the grouted ground. Figure 5, which is given on the basis of table 2, illustrates the variation of permeability after grouting along with depth. The coefficient of permeability in the original soils have a range from 2.01 x IO5 to 4.91 x lOA cm/set which is good for permeability, in case of grouted ground, Site-l, Site-2 has a tendency with decreasing the permeability from 1.19 x 1O”‘to 7.34 x lo6 cm/set.

Figure 5. Comparison of permeability in original ground with that in grouted ground

In other words, high pressure jet grouting is sufficient to obtain the cut off effect in soils. But that of weathered rock were 2.25~ lo4 - 6.17~ lo5 cm/set, then the grouted ground was 5.48~ IO” cm/set, which means the lower cutoff effect in the Site-2. The characteristic of subsoil in three sites was composed of silty sand, sandy gravel and sand in detail, it represents the similar permeability in Figure 5, since it was deposit soil with the nearly same relative density. As shown in Figure 5(a), the coefficient of permeability in the soil cement wall at No.1 site is almost same as the coefficient of permeability in the original ground before grouting. It shows that the permeability was not lowered by the grouting method Therefore, because the cutoff effect of ground water could not be achieved in Site-l by the soil cement wall grouting method, boiling problems were continued during excavation even after grouting. Besides, in Site-2 and Site-3, in which the high pressure jet grouting was performed, the coefficient of permeability in the grouted ground was lowered considerably after grouting as shown in Figures 5(b) and

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S(c). Therefore, the cutoff effect of ground water during excavation could be achieved sufficiently by the high pressure jet grouting in these sites.

The coefficient of permeability of grouted ground shows the range of 10Jcm/sec in application of the double tube rod type grouting method, while that of permeability of the range of lO%m/sec show in application ‘of the triple tube rod type grouting method. Bell (1993) presented that the coefficient of permeability for sand and sandy gravel injected by cement has a range from 14’ to 10-‘cm/set. Consequently, As shown in Figure 6, the coefficient of permeability are compared double with triple tube rod in the grouted ground. In the case of Site-2, that shows the range of 1.1 lx IOe5 to 7.13~ 10”

cm/set and 2.67x 10M6 to 7.34x 10m6 cm/set in the site-3. On the basis of above results, the coefficient of permeability measured in these sites is extremely similar to the permeability presented by Bell. Therefore, cutoff of ground water would be effective during excavation in ground injected by cement with high pressure. Figure 6 shlows the comparison of the coefficient of permeability, which is measured in the ground injected by cement with both double and Triple tube rod type high pressure jets.

4

-6

8

-2 10

H p

14

16

18

20

‘We

lEXr6 3E-06 lE-05 3E-05 O.ooo1 0.0003

Coefficient of PermeabiIity(an/s)

Figure 6. Comparison of permeability in grounds grouted with high pressure jets by different rod types

The coefficient of permeability for the ground grouted by the triple tube rod type has a lesser value of lO%n/sec than that of the doubie tube rod type. Therefore, the triple tube rode type is more effective to cutoff ground water than the double tube one.

CONCLUSIONS

Ground improvement and cutoff treatment of ground water have been performed to excavate cohesiontess soil with high ground water level and high permeability. Soil cement columns are made in the cohesionless soil injected by cement with some pressure. There are two kinds of grouting methods; One is the grouting method, in which the soil is mixed in ground by cement with relatively low pressure jet. The other is the grouting method in which the cement is injected in ground with high pressure jet. The effect of grouting on ground improvement and cutoff of ground water is investigated by field tests on the grouted ground and

laboratory tests on cores sampled from the grouted ground. The following conclusions could be obtained; 1) The strength of the grouted ground is affected considerably by the water content of original grotmd, which is related to existence of ground water level. 2) The high pressure jet grouting method is more effective than the low pressure grouting method to cutoff ground water behind temporary retaining wall. 3) Gn application of high pressure jet grouting, the tripIe tube rod type is more effective to cutoff groundwater than the double one.

REFERENCES

Bell, A. L., (1993) “Jet Grouting”, Ground Improvement, Edited by Moseley, M. P., CRC Press, Inc. pp.149 - 174.

Hong. W. P., Lim. S. B. and Kim. H. T. (1992). “Report on Safety Check for Excavation Works of Jahng-Hang Station in Ilsan City”, Korean Society of Civil Engineers.

Hong. W. P., Yun. J. M. and You. S. K. (1996). “A Case Study of Soil Improvement Constructed by High Pressure Jet Grouting Method ( I )“, Journal of Korean Geotechnical Society, Vol 12, No.3.pp.48-51.

Houlsby, A. C. “Engineering in Rocks Masses”, Edited by Bell, F. : with specialist contributions, Cha 17, pp.335.

Yoshiomi Ichihashi, Mitsuhiro Shibazaki, Hiroaki Kubo, Masahiro Iji and Akira Mori (1985), “Jet Grouting in Airport Construction”, Grouting, Improvement Soil and Geosynthetics Edited by R. H. Borden, R. D. Holtz and I. Juran, ASCE, Vol. 1, pp. 182 - 193.

Gohichi Miyasaka, Yutaka Sasaki, Toshiaki Nagata, Mitsuhiro Shibazaki, Masahiro Iji and Masami Yoda (1986). “Jet Grouting for a Self-Standing Wall”, Grouting, Improvement Soil and Geosynthetics Edited by R. H. Borden, R. D. Holtz and I. Juran, ASCE, Vol. 1, pp.144 - 155.

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